1374 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
are often difficult to separate. The pre-treatment process
involves separating the REEs from other elements and
impurities so that they can be extracted and purified more
efficiently. For instance, roasting step is required to break
down the fluorocarbonate structure. Roasting temperature
varies with the ore type. Bastnaesite is generally oxidised to
form oxyfluorides while CO2 gas is released after roasting
at 400–500 °C. Subsequently, oxyfluorides are turned into
acid-soluble oxides and insoluble trifluorides while releas-
ing HF gas at temperature of 500–700 °C. After that, Ce
is oxidised to produce insoluble cerianite at 700–1000 °C,
which can be separated from other REEs in the subsequent
leaching process (Sinclair, Baek et al. 2017). Figure 2 rep-
resents a comparison of two types of pre-treatments on the
same ore source for scCO2 extraction of REEs. Cerium is
unique amongst the REEs due to its potential to be oxi-
dised to the 4+ oxidation state, thereby simplifying some
separations.
Roasting and NaOH digest were the commonly used
industrial pre-treatment procedures. Sinclair et al. applied
a roasting pre-treatment of bastnaesite concentrate at 730
°C for 3 hours to break down the fluorocarbonate struc-
ture to enable contraction and increase solubility of REEs
(Sinclair, Baek et al. 2017). Similarly, the NaOH digest
also breaks down the structure to enable contraction and
increase solubility of REEs however it produces an acid-
soluble rare earth hydroxide. The NaOH digest was able
to achieve a greater extraction efficiency for all REEs. This
may be due to the faster reaction rate of NaOH digested
material. However, it is important to determine overall per-
formance of both pre-treatment options, including envi-
ronmental impact, infrastructure and operational cost.
Water and pH
For samples where the REO is contained within a solid,
water molecules in the system can act as modifiers to accel-
erate desorption of metal chelates from a solid sample. This
process improves the extraction efficiency through increas-
ing the dissolved metal chelate complexes. However, excess
water has been noted to decrease extraction efficiency as
water molecules will form stable adducts with metal che-
lates, making them less amendable to extraction (Ding, Liu
et al. 2017).
Excess water was found from the reaction of the com-
plex and metal oxides during the scCO2 extraction process
conducted by Shimizu et al. Evidently, water saturation
comes from preparing the chelating complex as shown in
Equation 1 with HNO3 containing a mixture of HNO3
and H2O. When extraction occurs, the water separates and
forms small water droplets in the scCO2 extraction system.
The metal ions become trapped in these droplets, reduc-
ing extraction efficiency (Shimizu, Sawada et al. 2005).
Shimizu et al. investigated a technique to prevent these
droplets from forming, and this involved the control of the
molecular ratio of TBP:HNO3:H2O within the complex by
using anhydrous TBP to prevent water precipitation. The
study compared the extraction efficiencies of REEs using
hydrated and anhydrous TBP, which found that the anhy-
drous TBP achieved slightly higher efficiencies with a more
significant impact on the Ce and La (Shimizu, Sawada et
al. 2005).
Rare earth element extraction benefits from higher acid
content within the system as it promotes the formation
of rare earth nitrate complexes which positively impacts
extraction efficiency (Yao, Farac et al. 2018). Furthermore,
50
60
70
80
90
100
La Ce Pr Nd
REE
Roasted
NaOH digest
Figure 2. Effect of pre-treatment techniques on the extraction efficiencies of REEs by scCO
2 extraction
(Chart plotted using data from (Sinclair, Baek et al. 2017))
Recovery
(%)
are often difficult to separate. The pre-treatment process
involves separating the REEs from other elements and
impurities so that they can be extracted and purified more
efficiently. For instance, roasting step is required to break
down the fluorocarbonate structure. Roasting temperature
varies with the ore type. Bastnaesite is generally oxidised to
form oxyfluorides while CO2 gas is released after roasting
at 400–500 °C. Subsequently, oxyfluorides are turned into
acid-soluble oxides and insoluble trifluorides while releas-
ing HF gas at temperature of 500–700 °C. After that, Ce
is oxidised to produce insoluble cerianite at 700–1000 °C,
which can be separated from other REEs in the subsequent
leaching process (Sinclair, Baek et al. 2017). Figure 2 rep-
resents a comparison of two types of pre-treatments on the
same ore source for scCO2 extraction of REEs. Cerium is
unique amongst the REEs due to its potential to be oxi-
dised to the 4+ oxidation state, thereby simplifying some
separations.
Roasting and NaOH digest were the commonly used
industrial pre-treatment procedures. Sinclair et al. applied
a roasting pre-treatment of bastnaesite concentrate at 730
°C for 3 hours to break down the fluorocarbonate struc-
ture to enable contraction and increase solubility of REEs
(Sinclair, Baek et al. 2017). Similarly, the NaOH digest
also breaks down the structure to enable contraction and
increase solubility of REEs however it produces an acid-
soluble rare earth hydroxide. The NaOH digest was able
to achieve a greater extraction efficiency for all REEs. This
may be due to the faster reaction rate of NaOH digested
material. However, it is important to determine overall per-
formance of both pre-treatment options, including envi-
ronmental impact, infrastructure and operational cost.
Water and pH
For samples where the REO is contained within a solid,
water molecules in the system can act as modifiers to accel-
erate desorption of metal chelates from a solid sample. This
process improves the extraction efficiency through increas-
ing the dissolved metal chelate complexes. However, excess
water has been noted to decrease extraction efficiency as
water molecules will form stable adducts with metal che-
lates, making them less amendable to extraction (Ding, Liu
et al. 2017).
Excess water was found from the reaction of the com-
plex and metal oxides during the scCO2 extraction process
conducted by Shimizu et al. Evidently, water saturation
comes from preparing the chelating complex as shown in
Equation 1 with HNO3 containing a mixture of HNO3
and H2O. When extraction occurs, the water separates and
forms small water droplets in the scCO2 extraction system.
The metal ions become trapped in these droplets, reduc-
ing extraction efficiency (Shimizu, Sawada et al. 2005).
Shimizu et al. investigated a technique to prevent these
droplets from forming, and this involved the control of the
molecular ratio of TBP:HNO3:H2O within the complex by
using anhydrous TBP to prevent water precipitation. The
study compared the extraction efficiencies of REEs using
hydrated and anhydrous TBP, which found that the anhy-
drous TBP achieved slightly higher efficiencies with a more
significant impact on the Ce and La (Shimizu, Sawada et
al. 2005).
Rare earth element extraction benefits from higher acid
content within the system as it promotes the formation
of rare earth nitrate complexes which positively impacts
extraction efficiency (Yao, Farac et al. 2018). Furthermore,
50
60
70
80
90
100
La Ce Pr Nd
REE
Roasted
NaOH digest
Figure 2. Effect of pre-treatment techniques on the extraction efficiencies of REEs by scCO
2 extraction
(Chart plotted using data from (Sinclair, Baek et al. 2017))
Recovery
(%)